The polyamides prepared by direct hydrolytic polymerization of amino nitrites often comprise portions of incompletely hydrolyzed intermediates. These intermediates reduce the molecular weight increase and may have adverse effects on the quality of the polyamide. Their presence is caused inter alia by incomplete removal of the ammonia produced in the reaction from the polymer melt.
A possible remedy in the case of a continuous or batchwise reaction is to use the precursor water in amounts which are much higher than stoichiometric in order to shift the reaction equilibrium to the required product side and/or to increase the reaction times, which is overall an energetically unfavorable procedure and may increase the proportion of unwanted byproducts or decomposition products. Another possible measure is to carry out the reaction in several reaction stages, consequently making the process technically elaborate.
A conventional industrial solution is a single-phase procedure for the hydrolysis and prepolymerization. However, this requires high operating pressures and pressure equipment suitable therefor, in order to keep volatile componentsxe2x80x94especially ammoniaxe2x80x94in solution.
For heat and mass transfer in the single-phase procedure a plurality of separate items of apparatus is required for heat exchange and mixing.
The aim of a novel process should be to keep the range of residence times for the reaction narrow so that non-uniform reaction times, which have an adverse effect on product quality, do not occur.
It is an object of the present invention to develop a process for preparing polyamides which, because the costs of starting materials and energy are less and the complexity of the apparatus is reduced, operates more cost-effectively and, at the same time, increases the conversion of intermediates through an advantageous process control with improved removal of low boilers.
We have found that this object is achieved by a process for preparing polyamides, their oligomers or mixtures thereof, where appropriate with other reaction products, by reacting amino nitriles and, where appropriate, other polyamide-forming monomers and/or oligomers with water in a reactive distillation, wherein the reaction product is discharged from the bottom of the reactive distillation apparatus, and ammonia which is produced and, where appropriate, other low molecular weight compounds which are produced, and water, are taken off overhead.
The process is preferably carried out continously.
Reactive distillations per se are sufficiently well known from the area of low molecular weight industrial chemistry. By contrast, there are few descriptions of them in the area of high viscosity systems, in particular of polymerizations. U.S. Pat. No. 3,900,450 describes, for example, reactive distillation to prepare nylon 6,6.
The object is achieved by combining the reaction and thermal separation technique in a single apparatus (=reactive distillation), characterized by a two-phase procedure during hydrolysis and polymerization. This design of process is referred to as reactive distillation hereinafter. Examples of equipment used are columns with or without internals or bubble columns.
In one possible procedure, for example, amino nitrile and water are fed into the upper half of the column. The low boilers produced in the reaction (ammonia and water) can then be enriched in the top of the column and taken off, while the high boilers in the bottom product comprise the required product of oligomers and polyamide.
This integrated process control with continuous product removal results in ideal, parallel heat and mass transfer with higher exergetic efficiency which is moreover characterized by rapid heating of the precursors and homogeneous mixing thereof. The reaction can be carried out under autogenous pressure.
For the present reaction system, the countercurrent flow of prepolymer and the reaction product ammonia, combined with the continuous removal of ammonia via the overhead product from, for example, column or bubble column, ensures very low ammonia contents in the products of the apparatus which contain amino nitrites substantially converted into required products.
It has been found that the process according to the invention results in higher conversions to required product than without continuous removal of ammonia via the overhead product, which shortens the reaction time and reduces the formation of unwanted secondary components.
The reaction can be assisted by employing any suitable catalysts which speed up the hydrolysis and/or condensation. Preferred catalysts are those which either can be introduced in solid form and consequently easily be removed from the required product, or else are present as coating on parts of the column.
The invention relates to a, preferably continuous, process for the hydrolytic conversion of amino nitrites into polyamide and/or its precursors and, where appropriate, further polyamide-forming mono- and oligomers into polyamide.
It is possible to employ as reactive distillation apparatus all suitable apparatuses which allow the reactants to react and the ammonia which is produced and, where appropriate, water to be discharged as gases. The reactive distillation apparatus preferably has a plurality of theoretical separation stages. In a preferred embodiment of the invention, a plate column, bubble column or dividing wall column is employed as reactive distillation apparatus.
If plate columns are used, the amino nitrile is preferably metered onto an intermediate plate in the upper part of the column. The amino nitrile then flows under gravity downwards through the apparatus and, during this, reacts continuously with water. The resulting ammonia continuously rises upward, owing to its volatility, and is rectified.
The precursor or mixture of precursors is preferably metered in as a liquid, or part is fed in as a liquid and the remainder, e.g. steam, is fed as gas into the apparatus. The latter variant has the advantage that the vapor can additionally serve as energy carrier. Steam is frequently available as energy carrier at reasonable cost.
The apparatus is preferably designed so that the thermal equilibrium is substantially set up, depending on the residence time, on the real or theroetical plates inside the column employed.
If a dividing wall column or an apparatus operating on the principle of a dividing wall column is employed as apparatus, it is possible for unwanted or wanted components to be discharged as intermediate boilers and, if required, returned to the apparatus at another point. This procedure can reduce losses of starting materials.
The precursors can, if required, be preheated through the overhead condenser.
An outline of this type of the principle of the process according to the invention is shown in the drawing of FIG. 1: